1. Field of the Invention
This invention relates to a circular accelerator in which charged particles are accelerated by a radio-frequency voltage and from which the accelerated charged particles are extracted, for being used as a particle beam therapy system.
2. Description of the Related Art
In circular accelerators such as synchrotron, charged particles are circulated and accelerated, then, the charged particles which are accelerated to high energy are extracted from a circulating orbit, and the charged particles (also referred as a charged particle beam or a particle beam) are transported by a beam transportation system. The obtained charged particle beam is utilized in a physical experiment where a desired object is irradiated or is utilized as medical use such as cancer therapy. A synchrotron comprises a vacuum duct for circulating a charged particle beam for a long time; a group of magnets which generate a dipole magnetic field or a quadruple magnetic field for controlling a circulating orbit or the size of a charged particle beam; a radio-frequency cavity, which accelerates a beam by a radio-frequency voltage (also referred as accelerating voltage) which is synchronized with a circulating period; a radio-frequency generator which controls a radio-frequency voltage to be applied to the radio-frequency cavity; an injector which introduces charged particles to a vacuum duct; and an extracting device which extracts a charged particle beam from a circular accelerator. Among the above-mentioned constituent parts, the radio-frequency generator comprises a radio-frequency source which generates an accelerating voltage; a radio-frequency control device which controls a frequency of the radio-frequency and a voltage; and an amplifier which amplifies the generated radio-frequency.
A radio-frequency generator applies an accelerating voltage to a radio-frequency cavity, and an incident beam having uniform distribution in time forms a bunched particle beam on a stable acceleration region. While acceleration of a beam, a frequency of an accelerating voltage to be applied to a radio-frequency cavity is increased. In a synchrotron which is a kind of circular accelerator (a circular accelerator includes a cyclotron whose circulating radius becomes larger as the beam is accelerated, in addition to a synchrotron whose circulating radius is constant), in order to make a circulating radius of a beam constant, corresponding to a dipole magnetic field intensity of a bending magnet for forming a circulating orbit of charged particles, a radio-frequency generator controls an accelerating voltage frequency. When a beam is accelerated to the intended energy, at the final stage, an orbit of the beam is bent by an extracting magnet and the beam is extracted from the circular accelerator.
In general, charged particles in a circular accelerator circulate while betatron oscillation is performed centering on a design orbit. On this occasion, the stability limit, called as the separatrix, exists. Charged particles within the stability limit, that is, the charged particles in a stable region circulate stably; however, charged particles which are beyond the stable region have the property such that the amplitude of oscillation is increased so as to be diverged. By utilizing this property, in order to extract charged particles, in conventional circular accelerators, by using a quadruple magnet, the tune which indicates betatron oscillation frequency per round of an accelerator (betatron number) is made close to be integer±⅓ and third order resonance is excited by using a sextupole magnet.
In extracting a particle beam, for example, a method, that is, the center momentum of charged particle beams as a group of charged particles which circulate is displaced by changing a frequency of a radio-frequency voltage to be applied to a radio-frequency cavity, the stable region of a betatron oscillation is narrowed so as to extract charged particles, is proposed (for example, JP2003-086399A). According to this method, as a beam is extracted corresponding to the amount of displacement of momentum, the beam is extracted while gradually changing a frequency of a radio-frequency voltage of a radio-frequency cavity.
Further, a method, in which electrodes which generate a radio-frequency voltage are provided in a circular accelerator in addition to a radio-frequency cavity, an amplitude of betatron oscillation is made increased by an electric field which is generated between the electrodes, without displacing the center momentum and with constant separatrix (the boundary between a stable region and a resonance region of betatron oscillation), so as to extract a charged particle beam by expelling a beam from a stable region to a resonance region is proposed (RF knockout method, JP5-198397A). According to this method, as the center momentum is not displaced, ideally, circulating frequency (center frequency) of a particle having the center momentum is constant; a radio-frequency signal to be applied to the electrode includes a frequency component which is synchronized with betatron oscillation. On this occasion, by considering such that in a precise sense, the tune of a particle has the continuous distribution, more effective extraction can be performed by widening the frequency band.
Recently, in a particle beam cancer therapy in which a circular accelerator is utilized, scanning irradiation method, in which a therapy aid (for example, bolus and collimator) for each patient is not necessary and a cancer site can be irradiated with high accuracy, is required. In a scanning irradiation, in general, beams are scanned in two dimensions by two dipole magnets (scanning magnets) of irradiation system and beams are scanned in the depth direction further by adjusting the energy so as to irradiate a target site. In a case where a scanning irradiation (Raster scanning irradiation), in which a beam having the same energy is continued to apply without stopping as a rule, a current strength of an irradiation beam having the high stability in terms of time is required. The higher the stability is, the easier the control of the irradiation dose is. Accordingly, the amount of a current of an irradiation beam can be increased, and the irradiation time can be reduced.